AS WITH A DESCRIPTION OF ANY MECHANICAL SYSTEM, we must first define a standard set of parameters, coordinates, and terms of reference. Global vehicle position and velocity are described in terms of three global axes defined by two SAE standards. SAE J670e, Vehicle Dynamics Terminology (SAE 1976), defines vehicle direction, motion, and travel with respect to three global axes as shown in Fig. 2.1 (Fig. 2 from SAE J670e). These are signed axes (i.e., there is a + and - direction), and one has to carefully coordinate the SAE J670e arrowheads to + signs when formally discussing vehicle motion. In Fig. 2.1, the +X axis is the forward longitudinal direction, the +Y axis is the right side direction, and the +Z axis is the down direction. These signed axes are very precisely defined in SAE J211, Instrumentation for Impact Test (SAE 1988). For clarity, the axes are clearly signed and labeled in Fig. 2.2.